The overall objective is to apply quantitative histochemical techniques to problems of renal physiology and biochemistry, both in the adult and in the developing animal. The purpose is to circumvent the heterogeneity of the kidney by direct analyses of the different segments of individual nephrons. (a) Many enzymes, characteristic of the adult kidney, are very low in the fetal kidney or at birth. One specific object is to determine the pattern of change in these enzymes along the nephron during development. (b) A second specific object is to contribute to the understanding of the locations and mechanisms of renal transport processes by examining the chemical changes along the nephron during active transport. What is proposed is a new approach to the study of renal transport processes. Identified portions of individual nephrons from kidneys engaged in active transport will be directly analyzed for (a) the transported substance, (b) a marker of glomerular filtrate such as inulin, and (c) substances that might be involved in the transport process, e.g. high energy phosphates, metabolic derivatives of the substance transported. It is believed that the new approach will complement older methods of study of kidney function. Because these studies can be made in both young and adult animals, they should permit an analysis of functional development not readily made by existing methods.